Metal fatigue is a problem common to just about everything that experiences cyclic stresses. Such problems are especially important in transportation equipment, such as aircraft, helicopters, ships, trains, cars and the like. Fatigue can also be present in other less obvious applications such as pressurized vessels, space vehicles, farm equipment, internal combustion engines, turbine engines, medical implants, industrial equipment, sporting equipment. Metal fatigue can be defined as the progressive damage, usually evidenced in the form of cracks that occurs to structures as a result of cyclic loading. This failure mode is not to be confused with a failure due to overload. The lower surface of an aircraft wing is a classical example of the type of loading that produces fatigue. The wing is subjected to various cyclic stresses resulting from gust, maneuver, taxi and take-off loads, which over the lifetime of a particular part eventually produces fatigue damage. Similarly, the pressurized envelope of an aircraft, including the fuselage skin and rear pressure bulkhead, are subject to a stress cycle on each flight where the aircraft interior is pressurized.
Fatigue can be a problem for holes and cutouts found in frames and bulkheads of fighter aircraft. Typically these structures have a variety of hole shapes and sizes (some non-round in shape) for the purpose of routing cables, wires, tubing and actuators through the aircraft. They can also serve as a means for allowing fuel flow from one bay to the next. In addition to serving as passageway holes they can also serve as lightening holes for reducing the weight of the structure. Lightening holes can also be found on bridges, trusses, construction equipment, semi-trailers and the like. Regardless of the function or purpose of the hole if they experience cyclic stresses they are subject to fatigue damage.
One problem inherent in fatigue damage is that it can be hidden since it generally occurs under loads that do not result in yielding of the structure. Fatigue damage is most often observed as the initiation and growth of small cracks from areas of highly concentrated stress. Undetected, a crack can grow until it reaches a critical size. At that point, the individual structural member can suddenly fail. Catastrophic failure of an entire structure can also occur when other members of the adjacent portions of the overall structure cannot carry the additional load that is not being carried by the failed structural member.
Automotive vehicles are also subjected to the damaging effects of cyclic stress. Vehicles driven on rough roads or off-road experience far more damaging loads on suspension, steering, wheels and the like than for those driven on smooth pavement. The firings of the pistons create cyclic loads on valves, valve guide holes, piston and connecting rod assembly, holes in and connecting both blocks and heads. Some fatigue is a result of high vibration of small stress. Metal covers surrounding and protecting mechanical assemblies may crack at holes due to vibratory loads. Holes created for the purpose of providing flow of lubricant or fluids are sometimes located in areas of high stress. These too, may experience fatigue damage.
Fortunately, failure due to the fatigue of an automotive component has generally less severe consequences than with an aircraft component failure. Even so, fatigue in automotive components has a large economic impact on the manufacturer because of the extent of the problem. Fatigue failures may show up only after the production of hundreds of thousands of units. Warranties work on that many vehicles can be very expensive and create a negative public image. Since fatigue damage usually occurs on highly stressed and typically more expensive parts these are the ones that are generally most costly to fix.
Large cylindrical and tubular rollers used in the manufacture of paper are perforated with thousands of holes allowing for the escape of liquids associated with the pulping process. The rollers used in paper production are basically rotating cylinders that are simply supported at both ends. The action of squeezing the pulp or pressing the paper under very high pressures creates bending stresses in the rollers. At the bottom of the roller tensile stresses are created and at the top of the roller compressive stresses are created. As the roller rotates through one complete turn the material experiences one cycle of alternating stresses; negative to positive. These applied cyclic stresses, coupled with the stress risers of many thousands of holes produce many potential fatigue damage sites on the rollers. Even non-perforated rollers experience fatigue because of the need for high speed, vibration free operation. Since the rollers typically rotate at a high velocity, any imbalance in the system can cause severe vibration. Typically, balance weights are attached to the roller through small bolt holes. The holes are subjected to the previously mentioned alternating stress cycle. Because the holes concentrate the stress they are a major source of fatigue cracks.
Orthopedic implants are subjected to repetitive cyclic loading from patient movements. Consequently, such implants are designed to resist fatigue. Orthopedic implants frequently include holes through which screws and other fasteners pass to attach the implant to the bone. The holes, while necessary for attachment to the bone, reduce the overall strength of the implant since they provide less cross-sectional area to accommodate the loads being transferred to the implant through the bone and also act as stress risers which reduce the ability of the implant to tolerate cyclic fatigue loading. The problem is particularly acute in trauma implants, such as bone plates, intramedullary nails and compression hip screws since these devices, in effect, stabilize broken bone fragments until healing occurs. Thus the loading imposed on the bone during the normal movements of the patient is immediately translated to the trauma implant which is then placed under greater stresses than a permanent prosthetic implant might be. The situation is aggravated if the bone does not heal as expected. In that case, the implant is required to accommodate not only greater stresses but also a longer cyclic loading period. Under such conditions, fatigue failure of the trauma implant is more likely.
Even stationary objects, such as railroad track or pressure vessels, may fail in fatigue because of cyclic stresses. The repeated loading from wheels running over an unsupported span of track causes fatigue loads for railroad track. In fact, some of the earliest examples of fatigue failures were in the railroad industry and in the bridge building industry. Sudden pressure vessel failures can be caused by fatigue damage that has resulted from repeated pressurization cycles. Importantly, government studies report that fatigue damage is a significant economic factor in the U.S. economy.
Fatigue can be defined as the progressive damage, generally in the form of cracks, which occur in structures due to cyclic loads. Cracks typically occur at apertures (holes), notches, slots, fillets, radii and other changes in structural cross-section, as at such points, stress is concentrated. Additionally, such points often are found to contain small defects from which cracks initiate. Moreover, the simple fact that the discontinuity in a structural member such as a fuselage or wing skin from a hole or cutout forces the load to be carried around the periphery of such hole, cutout or notch. Because of this phenomenon, it is typically found that stress levels in the structure adjacent to fastener holes, cutouts or changes in section experience stress levels at least three times greater than the nominal stress which would be experienced at such location, absent the hole, cutout or notch.
It is generally recognized in the art that the fatigue life in a structure at the location of a through aperture or cutout can be significantly improved by imparting beneficial residual stresses around such aperture or cutout. Various methods have been heretofore employed to impart beneficial residual stress at such holes or cutouts. Previously known or used methods include roller burnishing, ballizing, and split sleeve cold expansion, split mandrel cold working, shot peening, and pad coining. Generally, the compressive stresses imparted by the just mentioned processes improve fatigue life by reducing the maximum stresses of the applied cyclic loads at the edge of the hole. Collectively, these processes have been generically referred to as cold working. The term cold working is associated with metal forming processes where the process temperature is lower than the recrystallization temperature of the metal. Of all the methods used to cold work holes, presently the most widely used processes are the split sleeve process and split mandrel process. Together, these processes are referred to as mandrel cold working processes.
Cold working has shown to be effective on a wide variety of materials including cast iron, ductile iron, carbon steels, low alloy steels, intermediate alloy steels, stainless steels, high alloy steels, aluminum alloys, magnesium, beryllium, titanium alloys, high temperature alloys, bronze and the like. Historically, mandrel cold working was accomplished through strictly manual means. As an example, split sleeve cold expansion of holes is still done using hand-held hydraulic tools attached to air-actuated hydraulic power units. The variables involved in tool selection, implementation, and control of the cold expansion process require skilled operators to reliably produce properly treated holes. Unfortunately, the requirement of having a skilled operator to perform the task is a disadvantage in that it continuously presents the risk of improper or inaccurate processing. Also, such labor-intensive techniques effectively preclude automated feedback necessary for statistical process control. Although development of that process continues, the complexity of the split sleeve processes and the apparatus utilized presently precludes the widespread adoption of the process for automated fastening environments. The split mandrel process it at a similar stage of development; manually performed, but with some minor automation.
The mandrel cold working processes have a particular disadvantage in that they require precision in the size of the starting holes, usually in the range of from about 0.002 inch to about 0.003 inch in diametric tolerance, in order to achieve uniform expansion. Also, an undersize starting hole is required in that process, in order to account for the permanent expansion of the hole and the subsequent final ream that is necessary to remove both the localized surface upset around the periphery of the hole, as well as the axial ridge(s) left behind by the edges of the sleeve split or mandrel splits at their working location within the aperture, and of course, to size the holes. Moreover, treatment requires the use of two reamers; one that is undersized, for the starting hole diameter, and one which is provided at the larger, final hole diameter.
Another undesirable limitation of mandrel cold working processes is the requirement for, presence of, and residual effect of lubricants. For the split sleeve cold expansion process the starting hole must be free of residual lubricants (used for drilling) to prevent sleeve collapse during processing. A collapsed sleeve can be very difficult to remove and necessitates increasing the hole diameter beyond the nominal size, to remove the subsequent damage. The split mandrel process uses a liquid cetyl alcohol lubricant that must be cleaned from the hole after cold working, in order to ensure proper paint adhesion. In either case, the cold worked hole must be cleaned with solvents, in order to remove lubricants. Such chemical solvents are costly, require additional man-hours for handling and disposal, and if not effectively controlled during use or disposal, can have a deleterious effect on operators and/or the environment.
Still another limitation of the prior art mandrel cold working processes is their effect on the surface of the aperture being treated, i.e. the metal wall which defines the hole. The “split” in the split sleeve or the multiple splits in a split mandrel can cause troublesome shear tears in type 7050 aluminum, and in some other alloys. Shear tears, which are small cracks in the structural material near the split(s), are caused by the relative movement of the material near the split. Significantly, the increasing use of type 7050 aluminum in aircraft structures has created a large increase in the number of shear tears reported. Although such tears are generally dismissed as cosmetic flaws, they nevertheless produce false positives in non-destructive inspections for cracks.
Also, in the mandrel cold working processes, the sliding action of a mandrel produces a large amount of surface upsetting around the periphery of the hole, especially on the side of the structure where the mandrel exits the hole. In the split mandrel process, this effect is clearly seen, because of the direct contact of the mandrel with the aperture sidewall. The undesirable surface upset can increase the susceptibility to fretting, which may lead to a reduction in life for fastened joints. Additionally, surface upset in a stackup of structural layers can cause disruption of the sealant in the faying surface. To some extent the undesirable surface upset can be reamed out when sizing the final hole diameter, but at least some portion (and normally a substantial portion) remains.
Present methods of cold working holes and other cutouts using tapered mandrel methods, coining, punching, and such are not readily adaptable to automated fastening systems and other automated environments because of their complexity and bulkiness of equipment. Also, presently known methods used by others are not adapted to treat the entire periphery of non-circular cutouts, thus leading to fatigue life degradation. Finally, prior art countersink cold working methods require re-machining of the formed countersink to achieve the desired fastener flushness. Thus, the heretofore known processes are not entirely satisfactory because:                they often require mandrels, split or solid, and disposable split sleeves, which demand precision dimensions, which make them costly;        mandrels and sleeves are an inventory and handling item that increases actual manufacturing costs when they are employed;        “mandrel only” methods require a different mandrel for roughly each 0.003 to 0.005 inch change in hole diameter, since each sleeve is matched to a particular mandrel diameter, and consequently, the mandrel system does not have the flexibility to do a wide range of hole existing hole diameters;        each hole diameter processed with “mandrel only” methods requires two sets of reamers to finish the hole, one for the starting dimension and another for the final dimension;        mandrel methods rely on tooling and hole dimensions to control the amount of residual stress in the part, and therefore the applied expansion can be varied only with a change of tooling;        mandrel methods require some sort of lubricant; such lubricants, and especially the liquids, require solvent clean up;        splits in a sleeve or splits in a mandrel can cause troublesome shear tears, especially in certain 7000 series aluminum alloys;        the pulling action against mandrels, coupled with the aperture expansion achieved in the process, produces large surface marring and upsets around the periphery of the aperture;        split sleeve methods are not easily adapted to the requirements of automation, since the cycle time is rather long when compared with the currently employed automated riveting equipment;        mandrel methods are generally too expensive to be applied to many critical structures such as to aircraft fuselage joints, and to large non-circular cutouts;        mandrel methods have limited quality control/quality assurance process control, as usually inspections are limited to physical measurements by a trained operator.        
Thus, it would be desirable to provide an improved process for treating structures to improve their fatigue life, especially if such a process avoided many, if not most of the various drawbacks of prior art processes as just discussed above.